摘要:

AbstractAtomization experiments, motivated by the need to suppress misting of machining oils, were conducted on dilute solutions of polyisobutylene (PIB) in mineral oil using a coaxial air blast atomizer and an optical (Fraunhofer diffraction) particle sizer. Polymer concentrations (0.1 to 1.0 kg/m3) and molecular weight (1.0 to 2.2 million) were varied to determine their effects on aerosol drop‐size distributions. The pure oil and PIB ‐ oil solutions were atomized at air/liquid mass ratios near 10 and atomizing air velocities of 150–270 m/s, producing droplet‐size distributions with mass mean diameters (MMD) in the range of 7–15 μm for the pure oil. Under identical atomization conditions, the MMDs of PIB solutions were 20–200% higher than pure oil. These observed increases in MMD correspond to significant reductions in the fraction of droplets falling below 5 μm in diameter that constitute the misting problem in industrial machining applications. Observed effects of PIB on atomization are ascribed to the viscoelastic properties of PIB ‐ oil solutions as characterized by the elongational viscosity ηE. This relationship is examined by correlating the change in MMD caused by PIB addition with the stress‐dependent elongational viscosity of PIB‐oil solutions as predicted by the FENE‐P dumbbell kinetic theory. The increase in MMD due to PIB varies linearly with the predicted ηEat constant ato

ISSN:0001-1541    

DOI:10.1002/aic.690420502

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractRosensweig's (1979) analysis of the stability of gas‐fluidized magnetically stabilized fluidized beds (MSFBs) with voidage perturbations is modified to include terms dependent on the fluid density. This allows the stability analysis to be applicable to liquid‐fluidized MSFBs, which have potential use in liquid chromatography. Results show that in systems where the fluid density is not insignificant when compared to the solid‐phase density, the difference in density between the two phases should be used when normalizing the magnetization required to achieve stability. Comparisons between theory and experiment show that although the present analysis improves on Rosensweig's analysis, it still does not give reliable, quantitative predictions of stability behavior. It is suggested that for liquid‐fluidized MSFBs, stability to particle velocity perturbations, rather than to voidage perturbations, is more relevant to chromatographic applications wherein the purpose of magnetic stabilization is to prevent particle mixing as opposed to preventing the occurrence of bubbling and slugging observed in gas‐fluid

ISSN:0001-1541    

DOI:10.1002/aic.690420503

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA new technique was developed to measure the hydraulic permeability of reinforced gel membranes and allows calculation of the Darcy permeability (κ) of the gel. It was applied to agarose with concentrations ranging from 2.0 to 7.3%. To create membranes that would be thin enough to yield easily measured filtration rates at modest applied pressures and yet withstand handling, gels were cast on woven polyester meshes. The resulting membranes had thicknesses of 70–100 μm and a fractional open area of 0.32. To correct for the presence of the mesh, finite‐element solutions were obtained for the pressure field in the 3‐D region occupied by the gel. For the meshes used here, the hydraulic permeability of the reinforced membrane was calculated to be 0.47–0.55 times that for a layer of pure gel, the exact value depending on the thickness of the composite membrane. The principal determinant of κ was the agarose concentration, but there was a secondary effect of applied pressure. The Darcy permeability extrapolated to zero applied pressure (κ0) varied from 616 nm2for 2.0% agarose to 22 nm2for 7.3% agarose. At a given gel concentration, the value for κ0was as much as twice the value for κ measured at the maximum pressure difference of 20 kPa. This method can be adapted to a variety of other

ISSN:0001-1541    

DOI:10.1002/aic.690420504

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA theoretical model for the prediction of drop and bubble (fluid‐particle) breakup rates in turbulent dispersions was developed. The model is based on the theories of isotropic turbulence and probability and contains no unknown or adjustable parameters. Unlike previous work, this model predicts the breakage rate for original particles of a given size at a given combination of the daughter particle sizes and thus does not need a predefined daughter particle size distribution. The daughter particle size distribution is a result and can be calculated directly from the model. Predicted breakage fractions using the model for the air–water system in a high‐intensity pipeline flow agree very well with the available 1991 experimental results of Hesketh et al. Comparisons of the developed model for specific particle breakage rate with earlier models show it to give breakage‐rate values bracketed by other models. The spread in predictions is high, and improved experimental studies are recommended for verif

ISSN:0001-1541    

DOI:10.1002/aic.690420505

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractThe transport of water and pentane vapor in Langmuir–Blodgett (LB) films of arachidic acid is investigated. A quartz crystal microbalance is used to monitor the mass increases of the films upon exposure to the vapor as a function of the pressure. By examining films of different thicknesses, it is shown that it is possible to measure both surface adsorption and solubility. Diffusivity is calculated from the rate of uptake. Water and pentane have Fickian diffusion coefficients of 4 × 10−12cm2·s−1and 1 × 10−12cm2·s−1, respectively. The effect of both aging and annealing of the LB films on the vapor uptake is studied and it is shown that the uptake is sensitive to the stat

ISSN:0001-1541    

DOI:10.1002/aic.690420506

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA generalized parallel pore and surface diffusion model for multicomponent adsorption and liquid chromatography is formulated and solved numerically. Analytical solution for first‐ and second‐order central moments for a pulse on a plateau input is used as benchmarks for the numerical solutions. Theoretical predictions are compared with experimental data for two systems: ion‐exchange of strontium, sodium, and calcium in a zeolite and competitive adsorption of two organics on activated carbon. In a linear isotherm region of single‐component systems, both surface and pore diffusion cause symmetric spreading in breakthrough curves. In a highly nonlinear isotherm region, however, surface diffusion causes pronounced tailing in breakthrough curves; the larger the step change in concentration, the more pronounced tailing, in contrast to relatively symmetric breakthroughs due to pore diffusion. If only a single diffusion mechanism is assumed in analyzing the data of parallel diffusion systems, a concentration‐dependent apparent surface diffusivity or pore diffusivity results; for a convex isotherm, the apparent surface diffusivity increases, whereas the apparent pore diffusivity decreases with increasing concentration. For a multicomponent nonlinear system, elution order can change if pore diffusion dominates for a low‐affinity solute, whereas surface diffusion dominates for a high‐af

ISSN:0001-1541    

DOI:10.1002/aic.690420507

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractThe demulsification techniques investigated use preferential surface wetting to allow separation of oil and water phases in ultrafiltration and microfiltration membranes. A hydrophobic membrane allows the permeation of an oil phase at almost zero pressure and retains the water phase, even though the molecular weight of the water molecule (18) is much smaller than that of the oil molecule (198 for tetradecane, used in this study). Hydrophobic membranes having pore sizes from 0.02 to 0.2 μm were tested for demulsification of water‐in‐oil emulsions and water/oil/solid mixtures. The dispersed (aqueous)‐phase drop sizes ranged from 1 to 5 μm. High separation rates, as well as good permeate quality, were obtained with microfiltration membranes. Water content of permeating oil was 32–830 ppm depending on operating conditions and interfacial properties. For emulsions with high surfactant content, simultaneous operation of a hydrophobic and hydrophilic membrane, or simultaneous membrane separation with electric demulsification was more efficient in obtaining complete phase s

ISSN:0001-1541    

DOI:10.1002/aic.690420508

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractImprovements in bioprocess performance can be achieved by genetic modifications of metabolic control structures. A novel optimization problem helps quantitative understanding and rational metabolic engineering of metabolic reaction pathways. Maximizing the performance of a metabolic reaction pathway is treated as a mixed‐integer linear programming formulation to identify changes in regulatory structure and strength and in cellular content of pertinent enzymes which should be implemented to optimize a particular metabolic process. A regulatory superstructure proposed contains all alternative regulatory structures that can be considered for a given pathway. This approach is followed to find the optimal regulatory structure for maximization of phenylalanine selectivity in the microbial aromatic amino acid synthesis pathway. The solution suggests that from the eight feedback inhibitory loops in the original regulatory structure of this pathway, inactivation of at least three loops and overexpression of three enzymes will increase phenylalanine selectivity by 42%. Moreover, novel regulatory structures with only two loops, none of which exists in the original pathway, could result in a selectivity up to 95

ISSN:0001-1541    

DOI:10.1002/aic.690420509

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractA systematic and complete method to design robust predictive controllers for unconstrained linear systems is proposed. The synthesis procedure is based on rigorous theoretical foundations, without resorting to approximations or ad hoc design guidelines, yet it remains a viable tool for practical application. A significant feature is that the robust predictive controller retains the servo performance of a nominal predictive controller designed using conventional methods. In addition, the robust predictive controller can be designed to guarantee perfect steady‐state rejection of asymptotically constant disturbances. The robust design method is developed for systems affected by unmodeled dynamics and is based on solving a discrete‐time model‐matching problem. It is shown that the robust controller can be classified legitimately as a predictive controller because it minimizes the same performance functional as the nominal predictive controller. An illustrative design example is

ISSN:0001-1541    

DOI:10.1002/aic.690420510

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY

摘要:

AbstractTwo critical decisions must be made in the charging sequence of batch reactors: target setting and alarm generation. A number of statistics‐based strategies are proposed in this article to perform these tasks. The off‐line and on‐line target setting procedures developed in this work can be adopted to increase the profit margin of any given batch process without sacrificing reliability. A synthesis method for building optimal alarm logic is also described in detail. Monitoring systems constructed according to this suggested approach are effective in reducing the probability of undetected faulty batches. Extensive simulation studies show that the proposed strategies are suitable for application of manufacturing high‐value‐added products, which is a prevailing practice in batch

ISSN:0001-1541    

DOI:10.1002/aic.690420511

出版商:American Institute of Chemical Engineers    

年代:1996

数据来源: WILEY